In recent years, it has been proposed in the art to use a high dielectric constant material (hereinafter referred to as a “high-k material”) in the gate insulating film of a field-effect transistor in order to prevent the decrease in the driving current while suppressing the increase in the gate current. Specifically, it has been proposed in the art to use hafnium oxide (HfO2: relative dielectric constant ∈=30), zirconium oxide. (ZrO2: relative dielectric constant ∈=25), or the like, as the material of the gate insulating film so as to maintain a desired thickness of the gate insulating film, thereby reducing the leak current (see, for example, Japanese Laid-Open Patent Publication No. 2000-49349).
When hafnium oxide, for example, is used as the material of the gate insulating film, a silicate (hafnium silicon oxide) is formed at the interface between the silicon substrate and the gate insulating film. Specifically, when a heat treatment is performed after a gate insulating film made of a high-k material such as hafnium oxide or zirconium oxide is formed on a silicon substrate, the gate insulating film reacts with the underlying silicon substrate, whereby an insufficiently-oxidized silicate, which is a compound between the material of the gate insulating film and silicon, is formed near the surface of the silicon substrate. Where a hafnium oxide film is used as the gate insulating film, insufficiently-oxidized hafnium silicon oxide is formed as the silicate. Moreover, where a zirconium oxide film is used as the gate insulating film, insufficiently-oxidized zirconium silicon oxide is formed as the silicate.
Wet etching using an aqueous hydrogen fluoride solution is suitable for etching away a hafnium oxide film, a zirconium oxide film, or the like, as with a silicon oxide film. This is because an aqueous hydrogen fluoride solution does not substantially etch the silicon substrate.
However, it is difficult to remove a silicate such as hafnium silicon oxide or zirconium silicon oxide by a wet etching method using hydrogen fluoride, or the like. Specifically, the etching rate of wet etching using hydrogen fluoride, or the like, for hafnium silicon oxide or zirconium silicon oxide is about 1/10 to 1/30 that for hafnium oxide or zirconium oxide. Thus, when an insulative metal oxide film such as a hafnium oxide film is used as the gate insulating film, an insufficiently-oxidized silicate layer such as a hafnium silicon oxide film is formed at the interface between the silicon substrate and the gate insulating film, and it is difficult to remove the silicate layer by a wet etching method using hydrogen fluoride, or the like. As a result, the unremoved silicate remains on the surface of the silicon substrate when forming the gate electrode structure. Thus, a plurality of minute field-effect transistors, which are supposed to be electrically separated from one another, are electrically shorted with one another. Moreover, through a heat treatment step after the formation of the gate electrode structure, the silicate remaining on the surface of the silicon substrate is implanted into the silicon substrate as an impurity, thereby forming a level due to the impurity, which adversely influences the semiconductor substrate.